8.1.2.4 Sunspot Oscillations

Contributor: J. Staude, Solar Observatory `Einsteinturm'

Scientific Justification

MAG-waves (magneto-acoustic-gravity waves or magneto-atmospheric waves) are likely candidates for energy transport and heating in stellar atmospheres, moreover, they provide a method for sounding the structure of atmospheric and subphotospheric layers by seismology. Sunspots provide a unique example of a magnetized atmosphere where up-to-date observational techniques enable us to resolve both many details of such periodic disturbances and the basic atmospheric structure. Thus models of both MAG waves and atmospheres can be tested. Together with Soviet colleagues the present contributor published examples of both types of models for the sunspot umbra (Zugzda et al., 1987; Obridko and Staude, 1988). Being based on these models detailed calculations of the propagation of MAG waves through realistic models of an umbra have been performed which could help to resolve many problems in the interpretation of observations of umbral oscillations.

The observations show oscillations of velocity and intensity (that is, of temperature, density, etc.) in spectral lines formed at different height levels from the photosphere up to the chromosphere-corona transition region (CCTR). The data show closely packed, sharp resonance peaks of power in several period bands around 3 min, 5 min, and 20 min. In the chromosphere and CCTR the oscillations in the 3-min band dominate. So far observations in the CCTR have only been performed on the SMM (UVSP), and significant power in the 3-min period range has only been discovered in the CCTR above umbrae (Gurman et al., 1982). Simultaneous ground-based (photosphere, chromosphere) and space-borne (CCTR) measurements are rare.

Our model calculations suggested that a system of coupled resonators for various modes of MAG waves should act in the umbral subphotosphere and atmosphere. For instance, the 3-min oscillations should be due to a resonator for slow, quasi-longitudinal waves which are partly trapped in the photosphere and chromosphere. Our code provides information on the partial reflections and transmissions from each intermediate height level in the umbral model, moreover, on the resonance frequencies and on the height dependence of amplitudes and phases of oscillations of both velocity and thermodynamic quantitities. These theoretical predictions turned out to be in good agreement with the basic features of hitherto existing observations. More complete data are necessary for detailed tests as well as for improvements of the models and, more generally, for a selection from different competing models.

In order to cover a sufficiently large range of heights in the chromosphere and CCTR, lines formed at temperatures of , , and should be included into the programme. Moreover, co-ordinated ground-based observations, for instance, at the German vacuum telescopes at Tenerife should provide simultaneous measurements from the same sunspot, but from lower height levels (photosphere and lower chromosphere). The periods of oscillations expected are in the range 100-200 s, therefore the duration of a scan should not be much larger than , and the total observing duration should be at least 60 min. A sufficiently large scan area on the sunspot umbra is desirable in order to provide data for a diagnostic diagram for the first time. The requirement to observe many lines with very short dwell times is in conflict with the SUMER telemetry rate. The idea here is to use the Rear Slit Camera (RSC) to define the sunspot conditions in the red band and observe one UV line at a time. Together with a high telemetry mode (no CDS observation) this would provide adequate coverage.

Operational Sequence